The invention generally relates to compositions, articles, and methods for packaging oxygen-sensitive substances, especially food and beverage products. The invention is directed to oxygen barrier materials of the so-called active oxygen scavenger type. The active oxygen scavengers of this invention are condensation copolymeric substances which can be used for bottles and packaging. These compositions have an ability to consume, deplete or reduce the amount of oxygen in or from a given environment in the solid state at ambient temperatures. Formulations are disclosed which may be fabricated into clear plastic bottles suitable for recycle with other polyester bottles.
Plastic materials have continued to make significant advancements into the packaging industry due to the design flexibility of their material and their ability to be fabricated in various sizes and shapes commonly used in the packaging industry. The deployment of plastic materials into films, trays, bottles, cups, bowls, coatings and liners is already commonplace in the packaging industry. Although plastic materials offer the packaging industry many benefits with an unlimited degree of design flexibility, the utility of plastic materials has remained inhibited in situations where barrier properties to atmospheric gases (primarily oxygen) are necessary to assure adequate product shelf life. When compared to traditional packaging materials such as glass and steel, plastics offer inferior barrier properties which limits their acceptability for use in packaging items that are sensitive to atmospheric gases, particularly when the exposure to the atmospheric gases will entail extended time periods. The packaging industry continues to seek packaging materials which offer the design flexibility of plastics with the inherent recycle advantage of plastics and at the same time have the barrier properties of glass and steel.
The packaging industry has developed technology to improve the barrier properties of plastic containers by developing multi-layer containers that offer mixed polymer layers. These laminated packaging containers offer improved barrier properties approaching, but not comparable to, those of glass and steel while sacrificing many of the recycling benefits associated with single layer containers such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) bottles. Furthermore, depending on the mixtures of polymers, copolymers, blends, etc., used in the layers, clarity of the layered container is often substantially diminished. Maintaining the proper balance of recyclability, barrier properties, and clarity is most critical in bottling applications. However, these are common concerns among a wide range of plastic packaging uses.
PET has made significant inroads into bottling and packaging applications at the expense of the use of glass containers but primarily in applications where the needs for barrier properties are modest. A significant example is the use of PET for soft drink bottles. However, PET barrier properties have limited its use in the packaging of oxygen sensitive drinks such as fruit juices and beer. The most common size for PET soft drink bottles is the two liter capacity bottle but one liter and three liter bottles are also frequently seen. The wall thickness of PET employed for these larger sized bottles provides an adequate oxygen barrier for such products. Bottling fruit juice and other products of similar oxygen sensitivity in large bottles with thick PET walls has recently been commercialized. The increased wall thickness is needed to improve the barrier properties of the container but has a negative impact on the economics of the container. The ratio of packaging material to package volume has limited PET bottles to multi-serve container uses for packaging of oxygen sensitive foods and beverages. As the oxygen sensitivity of the packaged product increases or as the size of the package decreases, at some point the ratio of packaging material versus package volume becomes prohibitive. When that occurs, the production and use of thick walled conventional PET bottles is no longer economically viable as the cost of the packaging is disproportionate to the value of the packaged product. The availability of beverages and food in single serve plastic bottles and packages is an important economic consideration particularly for use in unusual sales locations such as at special events, in stadiums or arenas, and in similar situations where the amount of product sold is often determined by how quickly the product can be transferred from a multi-serve container to a single/consumer serving. Often, the sale of beverages in single serve glass or metal containers is prohibited at such locations because of the possibility that the empty containers may be hurled as missiles by rowdy attendees at such events. Sale of beverages in single serve plastic bottles, however, is normally permitted in all situations.
One possibilty for extending the economic viability of packaging oxygen sensitive materials in smaller or individual serving containers is to decrease the thickness of the bottle wall so as to maintain the same proportion of packaging material to package volume as would be found for larger bottles. However, containers with thinner walls made from conventional bottling polyester permit more passage of oxygen to the packaged product than thick bottle walls. As such, the shelf life and other required features of the bottle would not be satisfactory. However, modified thin bottle walls which maintain or improve the oxygen barrier characteristics of conventional bottling polyester could provide an answer. The use of multi-layer bottles that contain an inner, sometimes sandwiched, layer of a second higher barrier polymer material compared to the outer polymer layers, is already commonplace. Typically the center layer is a high barrier polymer that exhibits barrier properties by slowing the permeability of oxygen through the container wall. Such a system would be categorized as a passive barrier. A common construction for such passive barriers would comprise inner and outer layers of PET with a center layer of ethylene-vinyl alcohol (EVOH) polymer. Another method for providing increased oxygen barrier properties is the incorporation into the bottle walls of substances capable of intercepting and scavenging oxygen as it attempts to pass through the walls of the container. This method also affords the opportunity to eliminate unwanted oxygen from the package cavity wherein said oxygen may have been inadvertently introduced during packaging or filling. This method of providing oxygen barrier properties where a substance consumes or reacts with the oxygen is known as an xe2x80x9cactive oxygen barrierxe2x80x9d and is a different concept from passive oxygen barriers which attempt to hermetically seal a product away from oxygen via the passive approach.
One method for use of active barriers would be to make a three layer bottle which actually appears to be a mono-layer bottle. In the three layer bottle, the inner and outer layers are made of the same generic family of polymeric materials. The method applies to many packaging articles, but in the case of a bottle, the construction would comprise two polyester layers sandwiching a middle layer having outstanding oxygen scavenging characteristics atypical of the outer polyester layers. When the middle layer is very similar to the outer polyester layers, the article appears to be only a single layer. Of course many options exist including the use of a relatively homogeneous mono-layer comprising oxygen scavenging copolymers.
Incorporation of an active oxygen scavenger into the walls of a bottle provides a very effective means for elimination or at least control of the amount of oxygen which reaches the cavity of the package. However, there are some exacting demands which are placed upon the active oxygen scavenging walls of the bottle. One consideration is that the relatively thin walls of the bottle should be of sufficient strength and rigidity to withstand the rigors of filling, shipping, and use by consumers. The oxygen scavenging capacity of the bottle walls should be of sufficient capacity to allow for adequate shelf life and normal product turnover intervals. Shelf life and turnover intervals require that the oxygen scavenging should occur for extended periods of time. Most packaged products are stored and transported at room temperature or under refrigeration which mandates the necessity for oxygen scavenging activity at these temperatures. Of course, the oxygen scavenger should exist as a solid at these temperatures so as to be shaped and formed into packaging articles, i.e., these storage and transporting temperatures must be below the glass transition temperature (Tg) of oxygen scavenging compositions. The preferred compositions would absorb oxygen at a rate faster than the permeability of oxygen through the packaging wall for the planned shelf-life of the packaged product while having enough capacity to remove oxygen from within the package cavity if necessary. In those applications requiring clarity, the packaging article should have optical properties approaching those of PET. Finally, the preferred thin walled bottles should be suitable for recycle with other polyester bottles. In order to be meaningful, the recycling must be conducted without the need for any special physical processing such as delamination or the need for any special chemical processing such as depolymerization. What is needed are oxygen scavenging compositions, methods for the production of said compositions and methods of using the compositions in packaging articles so as to satisfy all the demands as recited above.
A number of attempts have been made to prepare oxygen barrier and/or scavenging bottle walls. Some approaches have involved the incorporation of inorganic powders and/or salts into the bottle walls. Most of these systems have numerous shortcomings including poor clarity, poor processing properties, insufficient oxygen uptake, and non-recyclability. There have been numerous approaches involving the use of laminated structures. Most of these have at least one or several disadvantages and most also suffer from lack of recyclability. Satisfying the need for a strong, recyclable, clear, thin walled polyester bottle with commercial oxygen scavenging capacity has continued to be a subject of substantial technical and commercial interest.
One method proposed for extending the range of utility for PET bottles is the incorporation of oxygen scavenging substances into PET. Such incorporation would increase the oxygen barrier properties of the modified PET permitting thinner bottle walls which would be ideal for smaller containers, especially for the bottling of oxygen sensitive substances. Naturally, increasing the oxygen barrier properties of PET must be done without sacrificing the salient features and properties of PET. For the purposes of this invention the salient features and properties of PET include (1) transparency, (2) rigidity, (3) good passive oxygen barrier properties, (4) recycle capability, (5) reasonable cost, and (6) a long history of experience and use in the packaging industry. Thus, there were at least two separate considerations involved in development of materials and methods that could be used to improve the oxygen scavenging properties of PET. Firstly it was necessary to identify a list of materials which may possess high oxygen scavenging capacity so that only small amounts of such materials would be required for use in fabricated form. Logic dictated that use of the smallest amount of material would have the least impact on the existent salient features of packaging polyesters. However, other considerations had to be made in addition to oxygen scavenging capacity including such factors as cost, clarity, processability, recycling, etc. Secondly, it was necessary to devise a means for innocuously incorporating the more promising scavenging substances into the packaging and bottling polyesters to form desirable oxygen scavengers.
Applicants were able to satisfy both of these considerations by developing novel condensation copolymer compositions comprising predominantly polyester segments and a lesser weight percentage of oxygen scavenging hydrocarbon segments. The oxygen scavenging hydrocarbon segments need be present only in an amount necessary to provide the degree of oxygen scavenging capacity needed for the particular application and are comprised of polyolefin oligomer segments which have been incorporated into the copolymer. For this invention, predominantly polyester segments is defined as at least 50 wt % polyester segments based on the weight of the copolymer. Since the copolymers comprise mainly polyester segments, such as PET segments, the properties of the copolymers formed remain very similar to those of the precursor polyester, i.e., the unmodified or homopolymer polyester lacking the oxygen scavenging hydrocarbon segments. The oxygen scavenging ability of these novel copolymers is present at temperatures both above and below their glass transitions temperature (Tg). However, a significant advance in the state of oxygen scavenging art arising out of this invention is the ability of these compositions to scavenge oxygen at temperatures below the Tg, (i.e., in the solid state). The Tg""s of the novel compositions of this invention are typically above 62xc2x0 C. which means the copolymers can be made into or incorporated into packaging articles that have commercial oxygen scavenging capacity at ambient temperatures in the range of about 0xc2x0 C. to about 60xc2x0 C. Also, since the novel copolymers are comprised largely of polyester segments, bottles constructed comprising the novel copolymers are suitable for recycle with conventional polyester bottles from other sources and with no need for special processing. Applicants have also devised methods for making the novel copolymers and methods for their use in the fabrication of bottles and other packaging articles.
A search of prior art has turned up some background references. Among these references are U.S. Pat. Nos. 5,310,497, 5,211,875, 5,346,644 and 5,350,622 (Speer, et. al.) which disclose the use of poly(1,2-butadienes) as an oxygen scavenger. But there is no disclosure of the compositions of this invention nor any recognition of the desirability to disperse this oxygen scavenging capacity in a polyester in any manner, and certainly nothing to suggest its use as segments in a condensation copolymer system. Furthermore, these addition type polymers of Speer, et. al., disclose oxygen absorption only above the glass transition temperature of the polymer system. The Tg of the Speer, et. al. materials is well below the use temperature commonly employed for packaging. This is a severe limitation to the Speer, et. al. polymers as it excludes the possibility for fabrication of the polymers into rigid packaging articles having oxygen scavenging capacity. It is well understood by those of ordinary skill in the art that below the glass transition temperature the polymer is in a glassy or solid state which gives the container rigidity. Further, it is also understood by those of ordinary skill in the art that the permeability of oxygen significantly increases above the glass transition temperature of the polymeric material. Thus in those systems where oxygen absorption occurs above the glass transition temperature the utility of the material is offset in part or totally by the increase in permeability of the oxygen through the polymer system or in loss of rigidity (shape). Simple polybutadienes as high molecular weight addition polymers are, in general, non-rigid and ill-suited by themselves to be used as a packaging resin or incorporated as a component of a rigid PET bottle construction.
As an example of prior art directed to the use of butadiene based copolymers with PET in general, in Japanese patent document 59196323 (Nov. 7, 1984) enhanced impact resistance or mechanical properties have been disclosed for copolymers from hydrogenated hydroxy terminated polybutadiene with PET oligomers, phenol, and terephthalic acid dichloride. It is known that hydrogenation serves to eliminate or at least severely diminish the number of tertiary and secondary hydrogens present in the butadiene. As will be discussed later in this application, oxygen scavenging ability is related to the presence and availability of secondary and tertiary hydrogen atoms in a hydrocarbon substance. Hydrogenation of the unsaturation in the polybutadiene polyolefin would serve to eliminate most of the secondary and tertiary hydrogen atoms sites and render such a composition impotent in terms of oxygen scavenging potential. As such the absence of hydrogenation of polybutadiene oligomers in the copolymers of this invention is an important distinction over this prior art. Also, in Japanese patent document 59193927 (Nov. 2, 1984) reactive extrusion has been described for preparation of aminated hydrogenated polybutadiene with polyester under catalytic action. U.S. Pat. No. 5,244,729 discloses the use of PET-maleated polybutadiene as an adhesive (one of many examples) for vermiculite platelets dispersed in oriented PET or polypropylene to create passive barriers comprising vermiculite platelets. Such a dispersion would necessarily be opaque as a result of the specified particle sizes ranging from 0.1-5.0 microns which would interfere with transmission of visible light. Applicants disclose copolymers with very small polyolefin oligomer segments which maintain transparency. Furthermore, there is no apparent recognition of the active oxygen scavenging capability of the polybutadiene functionality alone or for its use without the vermiculite. Japanese patent document 56129247 (Oct. 9, 1981) discloses hydrogenated diene copolymer with PET as a nucleant for PET crystallization. Japanese patent document 7308358 (Mar. 13, 1973) discloses PET-polybutadiene together with triisocyanates as an adhesive for polyester fiber tire cords in natural rubber. None of these prior art references has disclosed or made obvious the copolycondensates of this invention nor their efficacy for solid state oxygen scavenging.
This invention provides novel compositions in the form of copolycondensates which are effective oxygen scavengers that can absorb oxygen at packaging temperatures which are below the glass transition temperature of the polymeric compositions when deployed in the walls of plastic bottles or when incorporated in other packaging materials such as films, cups, cartons, bottle cap liners, can liners, food bags, trays, and the like. In a series of embodiments of this invention, this has been achieved by the preparation of copolymers capable of absorbing oxygen in the solid state below their glass transition temperatures comprising predominantly polyester segments with a sufficient amount of polyolefin oligomer segments to achieve the oxygen scavenging capacity required.
Also disclosed are methods of preparation of the oxygen scavenging copolycondensates. In a preferred embodiment, the copolycondensates are prepared by reactive extrusion transesterification of polyester with polyolefin oligomers which have been functionally terminated with end groups capable of entering into polycondensations.
Also methods of protecting oxygen sensitive substance by appropriate packaging are disclosed in a series of embodiments wherein oxygen sensitive substances are packaged in a suitable article of manufacture comprising the copolymers described above in sufficient quantity to serve as an oxygen barrier.
In several embodiments of this invention plastic bottles of sufficient oxygen scavenging capacity are disclosed so as to permit bottling, transportation, storage and sale of oxygen sensitive substances such as fruit juice without the need for cooling or refrigeration.
Finally, several bottling embodiments of this invention disclose polyester bottles having commercial oxygen scavenging capacity which are suitable for recycle with other polyester bottles without the need for any special processing.